Photochromic polymer matrix

ABSTRACT

A fast-acting photochromic filter- i.e., a filter whose optical density in the visible light range of 4,000 to 7,500 Angstromswhich is room temperature stable, that is from at least about 20* C. to 80* C., consists of (1) a solid optically transparent polymeric matrix, (2) essentially free from noncombined or molecular oxygen, and preferably protected from diffusion of atmospheric oxygen, in which is (3) dispersed, in solid solution, at least one light absorbing compound having a polynuclear conjugated condensed aromatic ring structure, whose molecules are capable of assuming a metastable triplet state by a process of intersystem crossing (not necessarily of the moiety which assumes the metastable triplet state) and in the triplet state, because of closer triplet states, has energy absorption bands which occur at lower energies (larger wavelengths) than the singlet-singlet absorption for such molecules. A synergic component may also be used as a physically adjacent moiety or molecule in which intersystem crossing occurs more readily than in the light absorbing compound, and which transfers triplet energy to the photochromic moiety. The absorption of singlet-singlet activating energy can be in the synergistic moiety, for single energy transfer, or in the photochromic moiety itself, with transfer of the singlet energy to the synergic moiety, intersystem crossing therein, to triplet energy, and transfer back of the triplet energy to the photochromic moiety, or double energy transfer. Two or more photochromic compounds, with different absorption spectra block &#39;&#39;&#39;&#39;windows&#39;&#39;&#39;&#39; to give absorption over a desired range. Protective glass layers on each side of a photochromic matrix give physical protection, exclude oxygen, and may also be filters for desired ultraviolet and infrared ranges. Polymerization by ionizing radiation gives freedom from interferrants, such as catalysts residues. Evacuation during processing, or heating in vacuum after formation of the filter aids in removal of uncombined oxygen.

llnited States Patent Stamm et a1.

[451 ,llan. 1d, 1972 IPHUTUCHRUMIC PULYMER MATRIX [73] Assignee: American Cyanamid (Iompany, Stamford,

Conn.

[22] Filed: Oct. 21, 1968 [21] Appl. No.: 769,028

Related US. Application Data [63] Continuation-impart of Ser. No. 332,752, Dec. 23, 1963, abandoned, Continuation-in-part of Ser. No. 332,921, Dec. 23, 1963, abandoned.

[52] U.S.Cl ..350/l60,252/301.2,264/1,

156/99,156/100, 161/1, 161/192, 161/410 [51] lnt. Cl. ..G02c 7/10, G02b 5/28 [58] Field of Search ..252/300, 301.2; 264/1, 2, 28,

264/109; 156/99, 100; 161/408, 192, 409, 410,1; 350/160 P; 96/90 PC; 256/3012 Primary Examiner-William J. Van Balen Attorney-Samuel Branch Walker [57] ABSTMAQT A fast-acting photochromic filteri.'e., a filter whose optical density in the visible light range of 4,000 to 7,500 Angstromswhich is room temperature stable, that is from at least about 20 C. to 80 C., consists of l) a solid optically transparent polymeric matrix, (2) essentially free from noncombined or molecular oxygen, and preferably protected from diffusion of atmospheric oxygen, in which is (3) dispersed, in solid solution, at least one light absorbing compound having a polynuclear conjugated condensed aromatic ring structure, whose molecules are capable of assuming a metastable triplet state by a process of intersystem crossing (not necessarily of the moiety which assumes the metastable triplet state) and in the triplet state, because of closer triplet. states, has energy absorption bands which occur at lower energies (larger wavelengths) than the singlet-singlet absorption for such molecules. A synergic component may also be used as a physically adjacent moiety or molecule in which intersystem crossing occurs more readily than in the light absorbing compound, and which transfers triplet energy to the photochromic moiety. The absorption of singlet-singlet activating energy can be in the synergistic moiety, for single energy transfer, or in the photochromic moiety itself, with transfer of the singlet energy to the synergic moiety, intersystern crossing therein, to triplet energy, and transfer back of the triplet energy to the photochromic moiety, or double energy transfer. Two or more photochromic compounds, with different absorption spectra block windows" to give absorption over a desired range. Protective glass layers on each side of a photochromic matrix give physical protection, exclude oxygen, and may also be filters for desired ultraviolet and infrared ranges. Polymerization by ionizing radiation gives freedom from interferrants, such as catalysts residues. Evacuation during processing, or heating in vacuum after formation of the filter aids in removal of uncombined oxygen.

17 Claims, 2 Drawing Figures lPll-TQTUUHITROMTC POLYMER MATRIX CROSS REFERENCES TO RELATED APPLICATIONS This is a continuation-in-part of application, Ser. Nos. 332,752 and 332,921, both filed Dec. 23, 1963, and both abandoned in favor hereof.

This invention relates to chromatic aspects of metastable electronic states in polynuclear and condensed ring-conjugated aromatic carbocyclic and heterocyclic systems and devices for utilizing the extremely fast color changes, particularly the reversible color changes at high levels of illumination of certain photochromic compounds, which color changes result from optical absorption of energy by molecules occupying metastable electronic states of one or more molecular species. The color change resulting from absorption of radiation is commonly referred to as PHOTOCHROMISM, or sometimes, and less properly, phototropism.

The more common photochromic compounds known to the prior art have a photochromic characteristic resulting from a molecular rearrangement or shift which is necessarily comparatively slow. Some of the common types involve rupturing chemical bonds, or cis-trans, or syn-anti isomerism. The slowness of these reactions is attributable to the time required for the relocation of atoms or fragments of molecules which are massive on a molecular scale. These reactions are temperature dependent, have finite and sometimes large energies of activation, and may not be reversible at low temperatures. Because of the characteristics of the reactions involved, sometimes the photochromic materials are degraded during the process.

The compounds used in the present invention, which for purposes of conciseness, will be referred to as photochromic materials, sometimes abbreviated as PM, have their photochromic activity dependent upon a physical phenomena involving a triplet-triplet absorption. This process in turn involves only the shifting about of the electron cloud of the molecule, and hence is extremely rapid and has the following major advantages:

a. The color change induced by the action of light occurs very quickly (within several microseconds) if the exciting light is intense and has a fast rise time.

The optical density, sometimes abbreviated O.D., is the logarithm to the base it) of the ratio of incident light to transmitted light (OD. log (I,,/l) where l intensity of incident ray and l= intensity of transmitted ray. Another equivalent definition is DD. log,,,( l/T) where T is the transmittance (1/10).

b. The optical density change produced by the action of the light, in the preferred embodiments, varies approximately linearly with the intensity of the incident light at levels of light intensity prevailing in sunlight (2-3 calories/cmF/ minutes). As the incident light intensity increases, the fraction of photochromic molecules remaining in the ground state decreases, and as the number available for conversion to the triplet state decreases, the optical density approaches a limit asymptotically.

Also, when using flash excitation from air-filled flash lamps which take 40 microseconds to reach peak intensity, and have lifetimes (time during which the flash has intensity greater than one-third the peak intensity) of 50 microseconds for the flash, and which provide total intensity levels of 0.003 to 0.01 l cal./cm. /flash at the sample, the change in optical density produced at the wavelength of maximum color development is directly proportional to the intensity of the incident light. Thus, under such conditions of flash excitation, increasing the incident intensity by a factor of 2 will also increase the optical density by a factor of 2.

c. The color development attributable to component I in a given matrix can be synergistically both enhanced and augmented by a second component Il present in the same matrix. This is made possible by means of single and double energy transfers between unlike molecules in a solid matrix when the energy levels of molecular species ll bear a preferred relationship relative to those of species I.

. If the photochromic material is to be incorporated in an optically clear plastic matrix, and if it is desirable or necessary to dissolve the active material in the monomer and then polymerize the monomer to form the plastic matrix, the use of ionizing radiation instead of conven tional polymerization catalysts produces a superior photochromic polymeric filter.

e. In addition to the high speed of color development (microseconds) and the relatively rapid rate of color decay, certain of these materials also exhibit photochromism in the presence of sunlight. Thus, the range of usages is extensive and includes such applications as the following:

Steady-State illumination Flash Illumination Sunglasses, welding goggles Pr ts qnsse ns flasht z Skylights ness and chlorioretinal 1165i; 7 Automobile windows and Windshields by light from nuclear bursts:

Safety goggles for same Windows for buildings and dwdlinrs WW9??? wi qshie [Qt w planes for same Windows for space vehicles and aircraft Safety sight glasses for qb seryational type optical devices Recording tapes with fast readout (telescopes, binoculars, rar@e 'hitdr's i Paints and surface coatings for novelty effects Recording tapes with fast readour i Energy measuring devices Energy measuring devices Safety glasses for scientific and industrial use in working around intense light sources Safety glasses for scientific use in working aroupd intepse 'fii iiis ou rees of short duration Display windows for stores There have been several papers published which disclose color changes induced in organic molecules by the action of light and which require a metastable electronic state (triplet state) as the key to the mechanism. D. P. Craig and l. G. Ross, The Triplet-Triplet Absorption Spectra of Some Aromatic Hydrocarbons and Related Substances," J. Chem. Soc. 1954, 15894969 (London) disclose achieving this phenomenon of color development at 78 K. (liquid nitrogen temperature) with the active species in a rigid, organic glass when they irradiated the sample with light from a l ,000 watt quartz mercury arc. The substances studied which produced the effect were:

Color Developed where r,,, is the mean lifetime of the metastable state.

The mean lifetime is the time for the population to fall to the fraction l/e of the population at the initial state, in the absence of the production of new members of the population. e is the base of Naperian or natural logarithms, 2.71828. The value of He is 0.36787.

At a later date, G. Porter and M. W. Windsor The Triplet State in Fluid Media," Proc. Roy. Soc. A245, 238-258 (I958) established that momentary color changes could be induced in dilute solutions of these same molecules (and other similar molecules) by irradiating the solutions with a short, intense burst of light from a flash lamp. In solutions, at room temperature, the color developed generally lasted no longer than I to 5 milliseconds since the metastable (triplet) molecules react with each other quickly when they are free to migrate. The solutions are very dilute, and the light path so long that such color development is of interest principally as a scientific curiosity.

Because the triplet state is so reactive, it has been considered necessary to operate at low temperatures, 'such solutions-of liquid nitrogen, or at extremely dilute solutions such that the absorption spectra have more theoretical than real significance.

We have now found that the molecules of polynuclear-conjugated condensed aromatic-ring systems meeting requirements for useful absorption in the metastable states are stabil- Compounds having an unsubstituted ring system, herein shownby a specific example, are listed by the name and formula given in the Ring Index, second edition, A. M. Patterson, L. T. Capell and D. F. Walker, American Chemical Society, Washington, DC, I960, with the abbreviation RRI giving the revised Ring Index serial number. The abbreviation R.I.S. refers to the Ring Index Supplement, L. T. Capell and i D. F. Walker, American Chemical Society, Washington, DC, I963. A Roman Numeral is assigned for substituted compounds. Where a Roman Numeral or a RRI number is assigned, the empirical and structural formula is given but once.

Compounds illustrative of those useful in the present invention and which give preferred absorption in the visible light region and are shown by specific examples, are shown in the following Table I:

TABLE 1 Chemical name, revised ring Index N o. and Empirical Example 0. formula Structural formula 1, ltl-phenanthroline RRI 3480, Example 1 CnHrNz 2 N l 10 N 9 1, 7-phenanthr01lne RRI 3478, Example 2 CrzHrNz A Anthracene RRI 3618, Example 3 CHHm 8 9 l Phenanthrene RRI 3619, Example 4 CnHn Tlgefibenidine (4-aza'pyrene) RRI 6178, Examples CnH N Pyrene RRI 5262, Example 7 CnHro Chmnical name. rvvisvd ring lnvlux No, and Empirical Example No. formula Structural formula Benzukllxanthene RRI 5230, Exambles 8, 9 01011100 BenZoIb]naphtho[1,2-d]-thiophene RRI 4684, Ex- CuH S 2 ample 10.

DibenzolLhlquinoxaline RRI 5010, Example 11... CmHmNz 2 7H-benzlda1anthracen-7-ono I (from RRI 6261), CnHmQ Example 12.

Bonz[u]ucrldino RRI 5144, Examples 13, 14 CnHnN Begglbhcrldine RRI 5140, Examples 15, 16, 79, 911, CnHnN 10 11 12 1 Benzlcflacridine RRI 5148, Examples 17, 18 CnH N 2 Benz[b]acrldin-12(5H)-one II, Examples 19, 20, 98, CuHuNO Chemical name, revised ring Index No. and Empirical Example No. fernmh Structural formnln Benzolghilfluoranthene RRI 6002, Enmpe121. 1 CuHm Benz[a]anthracene RRI 5253, Example 22. CuHn Chrysone RRI 5254, Example 23 1 Cull" I!enzolblnaphtho[2,3-f][1,4]-d1azocine RIS 9111, CIIII|2N2 Example 24.

2-Aminochryseno III, Examplo 25 CuHuN Be3z[e]ucepl\enanthrylene RBI 6070, Example CaHm I1 enzohflpyrene RRI 6399, Examplc 27 CaHn Benzololpyrene RBI 6400, Examples 28 0 H Empirical Chemical name, revised ring Index No. and

\ formal:

Example 0 Structural fommlu Dibenz[a,h]acridine RRI 6298, Examples 40, 41, CzxHu Dlbenz[a,j1acridlne RRI 6296, Examphs 42, 43... CnH N Benzofighllperylene RRI 7036, Example. 44, 45, 0111112 In eno[1,2,3-cd1pyrem (orthophnnylono pynno) C1211;

d ILLS. 9508, Enmplls 46, 47.

Benzggblchrysene RBI 6379, llxunplu 48, 49, 80, CuHu Chemical name, revised ring Index No. and Empirical Example No. formula Structural formula Dibenz[a,c]anthracene RBI 6381, Example 50.. 02211 2 Dibenz[a,hlanthracene RRI 6383, Examples 51, C22Hu Plcono RRI 6384, Example 55 C ll 2 Dlbenzo[c,g]phonanthrene RRI 6887, Example 66. CzzHu 14 1 10-methylbenzo[g]chrysene V, Example 57 CzaHw 10-m0thylbenzo[g]chryscne-l,3,6-trinltrobenzane CgaIImNaOa complex VI, Example 58.

Chemical name, revised ring Index N o. and Empirical Example No. formula I Structural iorululu Coroueue RRI 7392, Examples 59, 00- l Cell-In 12 1 Dibenzo[def,p]chrysene RRI 7030, Example 6l. CZJHH Naphtholl,2,8,4-dei']-chryseno RRI 7026, Examples C2AH14 D1b0nzo[fg,op]-naphthuceno RRI 7022, Examples CZJHH Dlbenzo[b,def]chrysen0 RRI 7028, Example 66... C24Hu Begzolistlpentaphene RBI 7029, Examples 67, 68, I CnHu Chemical name, revised ring Index N o. and Empirical Example N o. formula Structural formula 2,3-Diphenylbenzo[l']quinnxallne VII, Example 70. C H1|N;

Dlbenzo[b,k]chrysene RBI 7002, Example 71.-.... 03H

Dibenz0[g,p]cln'yseno RBI 7010, Example 72. CuHn 3 Benzo[a]coronene ILLS. 9650, Examples 73, 74, CuHu 14 1 llillifia revised ring Index No. and Empirical e No. furmuln Structural formula Dibenzo[l1',rsn]pentapl1ene RBI 7381, Examples CuHu Benzoh'flchrysene picrate VIII, Example 77 Cz|HnNaO1 Naptho[2,3-a]coronene R.I.S. 9678, Examples 92, CuHu Acenaphtlwll,2-b1-benzoll'1qulnoxallne RRI 681G, (Jul-InNz Example 94.

l3enzola]uaphtl1o-[B,1,2-cde1nnphtlmceno RBI Cali" 7373, Example 95.

Trllgenzolbmpqrl-porylono RBI 7524, Example 0,.Hn

Chemical name, revised ring Index No. and Empirical Example No. formula Structural formula Dibenzo[a,g]comnene R18 9680, Example 104... C Hu Illustrative of other compounds giving useful spectra art the.

following:

TABLE 2.-ADDI'IIONAL REPRESENTATIVE COMPOUNDS WHICH EXHIBIT TT ABSORPTION Emplrlcal Chemical name, revised ring index number Formula Structural Formula Permidine RBI 3398 C EN; 2

Acenaphthennqulnone C12H|02 Acenaphthylqno RR! 3133 l. CnHn Fluoren-Q-one (RRI 3127) (Plronthniis it closoly CIZIIIO 0 related to RRI number). 8 9 1 Thioxanthen-Q-one (RRI 3607) C HaOI 0 1 Phenaleu-l-one hydrazone (RBI 3690) CuHnNz 1 2 NH -N" Empirical Chemical namo, revised ring index number Formula Structural Formula 12H-dibenzo[b,h]-iluorene RRI 6045, dibenzfimr Call 11 ll, rene (all isomers). 12 C l)ibeu7,[b,i]acridiuo RRI 6287, dibenzaerldino (all C21II13N 11 12 other isomers) (see Examples 42, 43ior [31, 140, 41

14 1 etc. for [a,h])- 10E N/ k/ Dibenzo[c,j]phenanthridine RRI 6290 (and c,i; Same 1 Tris[p-(dimethylamino)-phonyl]borane(III) .4 CzlHaoBNa Dibenzo[def,nmo]-el1rysene-6,12-dlone CzzHmOz (antlmnthrone) (RBI 7033).

Acenaphtho[1,2-b]benzo-[g]quinoxallne RRI 6813 C22H12N2 (see Example 94 for [1,2-b]-[f]isomer).

Dibenz[a,i]ant-hracene RRI 638 2, dibenzanthra- CzzHu cone (and other isomers) (see Example 50 for [a,c] Example 51 etc. for [a,h]).

])ihenzolb,g]phenanthrene RBI G380, dibenzo- CzzHu phenanthrene (and other isomers) (seo Example 66 for [c,g1).

2,5-(1i-2-naphthyl-1,3,4oxadlazole UzzIIuNzO Empirical Chemical name. revised ring index number Formulu Structural Formula ERI ()H N(UH:)

J 7 I tl ,13

Diacenaphth0[1,2b:1,2"d]thiophcuu (Di-1,8- (11411 8 naphthalcnethlophene) R R1 7122.

Rubicene RRI 7167 02011 4 8,llfi-p gmutlu'onediono (pymnthronc) (from RRI 030N140: O

Pyranthrene RRI 7626 O E 15 16 l Also included are the lower alkyl substituted derivatives formed by substituting one or more lower alkyl groups on the compounds listed above and listed in table I.

Some compounds have an absorption of the T'-T-type at least part of which is outside of the visible range and, hence, are primarily useful in the visible range as synergistic additives for absorption systems containing two or more absorbers. Others absorb in the visible range when activated but are more useful as synergistic compounds to extend the range or improve the efficiency of other compounds. The synergic additives may function as single or double energy transfer agents.

For fast-acting photochromic effects by triplet-triplet (T'T) absorption, the energy-absorbing electron system in the triplet configuration results from absorption of energy by a singlet configuration at a shorter activating wavelength, then intersystem crossing to a triplet configuration followed by energy absorption at longer wavelength. Often, the intersystem crossing efficiency of the molecular structure in which the triplet configuration is desired is low, so that a more effective intersystem crossing from singlet to triplet electronic configuration is achieved by using one molecular structure for achieving the intersystem crossing from singlet to triplet fol-' lowed by transferring the triplet energy to a molecular structure which has a comparatively long triplet lifetime and effective absorption at the desired wavelength. If the energy is absorbed as singlet energy in molecular structure D, and converted in D by intersystem crossing to the triplet state, and then transferred to structure A, wherein the filtered light is absorbed, the process is called single energy transfer. lf absorbed by A transferred to D as singlet energy, crossed in D to triplet energy, and then transferred back to A, the process is called double energy transfer.

The transfer of energy between triplet states was recognized at least as early as 1956 by Terenin and Ermolaev of the Academy of Sciences of U.S.S.R. An English translation of their paper appears in the Transactions of the Faraday Society 52, 1,042-1 ,051 (1956). Their work was in frozen solutions at about -l95 C. The transfer of singlet energy between anthracene and naphthalene structures, separate and linked by saturated chains of one, two and three carbon atoms is disclosed by Schnepp and Levy, 1. Am. Chem. Soc. 84 172-177 (1961). Work reported by Lamola, Leermakers, Byers and Hammond J. Am. Chem. Soc. 87 2,322-2,332 (1965) confirms the intramolecular transfer of energy.

Because of the complexity of the chemistry and quantum mechanics involved, a full list of the known compounds or additives which give useful absorption in the visible region, alone or in synergistic combination, would unduly expand this already prolix specification. Additionally, using the criteria herein expressed, other and new compounds can be synthesized.

IN THE DRAWINGS FIG. 1 is a schematic energy level diagram of a system involving a single molecular species.

FIG. 2 shows a useful filter system involving the present photochromic materials in a matrix as a filter, in conjunction with other filters.

FlG. l is a schematic energy level diagram showing the mechanism by which the metastable light absorption phenomena occur. As proved by quantum mechanics, molecules cannot have just any energy. Instead, only definite values of energy are possible, and these will be characteristically unique for any given molecular species. In FIG. 1, discrete electronic energy levels are depicted by the horizontal lines, and the transitions between energy levels are indicated by vertical or diagonal arrows up for absorption, down for emission. Full arrows are used for transitions which can be observed optically whereas dotted lines indicate radiationless transitions. At room temperature and below essentially all the molecules of the photochromic material (PM) in a sample are distributed among the low-lying rotational and vibrational sublevels (not shown in the diagram) of the lowest (or ground) electronic state (G).

Two series of electronic levels are shown. The series on the left consists of so-called singlet levels, S, S"....while that on the right consists of triplet levels, T, T, T". According to a spectroscopic rule enunciated by Hund, under normal circumstances transitions between levels of unlike multiplicity (singlet and triplet) are not probable and are said to be -forhidden."

Triplet State: A molecule whose electronic configuration is such that it has two unpaired electrons with their spins parallel is said, physically, to be in a triplet state. Chemically, such a molecule would be called a biradical and would be very reactive.

The spectroscopic notations singlet, doublet, triplet, etc., designate the number of separate electronic levels into which the one level will split when the molecule is subjected to the action of an intense magnetic field. A spectroscope can be used to confirm this existence.

The lowest electronic state in the group of triplets has an energy greater than that of the ground state and is generally a -metastable" state, i.e., one from which optical transitions by emission are improbable (forbidden) so that the lifetime of a molecule in such a state can be much greater than normal. Thus, instead of having mean lifetimes of IO seconds or less,

'which would be considered to be normal for any state other than the ground state, metastable states have means lifetimes of 10' to about 10 seconds or more. It should be noted that such metastable states can be reached only by an indirect route. Without being limited to the correctness of the scientific basis for the phenomena involved, it is postulated that, in order to achieve photochromism by a mechanism involving such a metastable electronic state, the following sequence of events probably occurs: W 1

a. The substance in the ground state (G) absorbs radiation having sufi'icient energy to raise the molecules to states which are one of a plurality of possible singlet levels, S, S", or 8'' (not shown), or higher.

b. In a condense phase, molecules in states S", S, or higher, will lose energy by radiationless exchanges and end up on the lowest vibrational level of the singlet state, S. This process is called interconversion."

c. From state S, molecules can emit light and fall back to the ground state by a process called fluorescence.

d. By the process of intersystem crossing," singlet molecules can also lose energy by radiationless exchange and end up in the metastable state T. Because of the long lifetime of this state, under certain conditions of illumination, the population of the metastable state can become greater'than that of the ground state, even at room temperature.

e. Once in the metastable state, molecules can emit light as 

2. A method of preparing a storage stable photochromic filter consisting of preparing a filter as in claim 1 and sealing the same between sheets of glass with a gaseous oxygen excluding seal, thereby excluding atmospheric oxygen.
 3. A method of preparing a photochromic filter comprising dissolving in a solvent (a) a polymer substantially free from oxygen and active centers capable of reacting with free radicals and biradicals and which polymer is solid at room temperature, (b) at least one photochromic material, the molecules of which have a polynuclear conjugated condensed aromatic ring system, which system has a set of electronic energy levels such that at least one of said levels is metastable and characterized by having strongly allowed transitions from the metastable level to higher electronic states whose energy spacings relative to the metastable state correspond to energies of photons capable of exciting a visual response in the retina of the human eye, namely whose wavelength is in the range of 4,000 to 7,500 A. units, and which metastable energy level is populated by producing a redistribution of the electrons such that the electronic configuration peculiar to the ground state is altered to the electronic configuration characteristic of the metastable state when the ground-state molecules are subjected to excitation by photons having energies greater than that corresponding to photons having a wavelength of about 6,000 A., (c) casting the solution into the desired shape for the filter, and (d) removing the solvent by evaporation, at least the last portions of the solvent being removed by vacuum and mild heating, thereby simultaneously removing oxygen, thus forming a photochromic filter.
 4. A method of preparing a storage stable photochromic filter consisting of preparing a filter as in claim 3 and sealing the same between sheets of glass with a gaseous oxygen excluding seal, thereby excluding atmospheric oxygen.
 5. A method of preparing a photochromic filter comprising (a) dissolving at least one photochromic material in a polymer forming monomer, (b) removing uncombined oxygen by subjecting to vacuum, and (c) while being retained free from uncombined oxygen, (d) subjecting to a polymerizing dose of high-voltage ionizing radiation, and heat, thereby polymerizing substantially all of the monomer, so that the filter is substantially free from uncombined oxygen, and has less than 1.5 percent residual monomer, said photochromic material having molecules which have a polynuclear conjugated condensed aromatic ring system, which system has a set of electronic energy levels such that at least one of said levels is metastable and characterized by having strongly allowed transitions from the metastable level to higher electronic states whose energy spacings relative to the metasTable state correspond to energies of photons capable of exciting a visual response in the retina of the human eye, namely whose wavelength is in the range of 4,000 to 7,500 A. units, and which metastable energy level is populated by producing a redistribution of the electrons such that the electronic configuration peculiar to the ground-state is altered to the electronic configuration characteristic of the metastable state when the ground-state molecules are subjected to excitation by photons having energies greater than that corresponding to photons having a wavelength of about 6,000 A.
 6. A method of preparing a storage stable photochromic filter consisting of preparing a filter as in claim 5 and sealing the same between sheets of glass with a gaseous oxygen excluding seal, thereby excluding atmospheric oxygen.
 7. The method of claim 5 in which the monomer is essentially pure methylmethacrylate.
 8. The method of claim 5 in which the monomer is methylmethacrylate which additionally has present from 1 to 5 percent by weight of ethylene glycol dimethacrylate.
 9. A storage stable photochromic filter effective at room temperature comprising a polymeric material solid at room temperature having dispersed therein at least (I) one photochromic material, the molecules of which have a polynuclear conjugated condensed aromatic ring system, which system has a set of electronic energy levels such that at least one of said levels is metastable and characterized by having strongly allowed transitions from the metastable level to higher electronic states whose energy spacings relative to the metastable state correspond to energies of photons capable of exciting a visual response in the retina of the human eye, namely whose wavelength is in the range of 4,000 to 7,500 A. units, and which metastable energy level is populated by producing a redistribution of the electrons such that the electronic configuration peculiar to the ground state is altered to the electronic configuration characteristic of the metastable state when the ground-state molecules are subjected to excitation by photons having energies greater than that corresponding to photons having a wavelength of about 6,000 A., and said polymeric material additionally having dispersed therein (II) an intensifying synergistic compound having an aromatic ring system, which intensifying compound a. has its lowest excited singlet above the lowest excited singlet level of the photochromic material; b. has its lowest triplet above the lowest triplet of the photochromic material and below the lowest excited singlet of the photochromic material; c. does not fluoresce; and d. has a high rate of crossing, (III) said photochromic material and said synergistic compound being protected from atmospheric oxygen by an uncombined oxygen resistant barrier.
 10. The photochromic filter of claim 9 in which the photochromic material has present at least two photochromic compounds in which absorption windows of one compound are blocked by another photochromic compound present.
 11. A storage stable photochromic filter consisting essentially of three layers, in order from a light source, (A) a glass filter transparent to ultraviolet light and which absorbs in the infrared region; (B) a photochromic filter as defined in claim 9; and (C) a glass filter which is substantially opaque to ultraviolet, deep red, and infrared radiation, and which reduces the intensity of the entire visible spectrum, but has its maximum transmission in the region of 5,400 to 5,500 A., and the human eye is most sensitive. namely
 12. A storage stable photochromic filter effective at room temperature comprising a polymeric material solid at room temperature and having less than about 1.5 percent residual monomer, and being substantially free from uncombined oxygen and active centers capable of reacting with free radicals and biradicals having disperseD therein a photochromic material, the molecules of which have a polynuclear conjugated condensed aromatic ring system, which system has a set of electronic energy levels such that at least one of said levels is metastable an d characterized by having strongly allowed transitions from the metastable level to higher electronic states whose energy spacings relative to the metastable state correspond to energies of photons capable of exciting a visual response in the retina of the human eye, anmely whose wavelength is in the range of 4,000 to 7,500 A. units, and which metastable energy level is populated by producing a redistribution of the electrons such that the electronic configuration peculiar to the ground state is altered to the electronic configuration characteristic of the metastable state when the ground-state molecules are subjected to excitation by photons having energies greater than that corresponding to photons having a wavelength of about 6,000 A., said photochromic material being protected from atmospheric oxygen by an uncombined oxygen resistant barrier.
 13. The photochromic filter of claim 12 in which the photochromic material comprises at least one material having a polynuclear conjugated aromatic system selected from the systems listed in table 1 and table 2, and the polymeric material solid at room temperature is selected from the group listed in table
 5. 14. The photochromic filter of claim 13 in which the photochromic material, at least in part, consists of benzo(a)coronene.
 15. The photochromic filter of claim 12 in which the uncombined oxygen resistant barrier consists of a first layer of glass on one side of the polymer layer which glass is a filter transparent to ultraviolet light and which absorbs in the infrared region, a second layer of glass on the other side which is substantially opaque to at least ultraviolet radiation, and an oxygen resistant seal around the edges of the glass layers to prevent diffusion of oxygen through the edges of the thus-formed filter.
 16. The photochromic filter of claim 15 in which said second layer is a glass filter which is substantially opaque to ultraviolet, deep red, and infrared radiation, and which reduces the intensity of the entire visible spectrum, but has its maximum transmission in the region of 5,400 to 5,500 A., to which the human eye is most sensitive.
 17. The method of reactivating a photochromic filter consisting of a. a polymer substantially free from oxygen and active centers capable of reacting with free radicals and biradicals and which polymer is solid at room temperature, b. in which is dissolved in solid solution at least one photochromic material, the molecules of which have a polynuclear conjugated condensed aromatic ring system, which system has a set of electronic energy levels such that at least one of said levels is metastable and characterized by having strongly allowed transitions from the metastable level to higher electronic states whose energy spacings relative to the metastable state correspond to energies of photons capable of exciting a visual response in the retina of the human eye, namely whose wavelength is in the range of 4,000 to 7,500 A. units, and which metastable energy level is populated by producing a redistribution of the electrons such that the electronic configuration peculiar to the ground state is altered to the electronic configuration characteristic of the metastable state when the ground-state molecules are subjected to excitation by photons having energies greater than that corresponding to photons having a wavelength of about 6,000 A., c. and which has been substantially inactivated by atmospheric oxygen diffusing thereinto d. comprising heating under high vacuum until substantially all uncombined oxygen has diffused out of said filter. 